How to open HTTP stream on iOS using ndjson

Does anyone have experience opening HTTP stream on iOS? I have tried multiple solutions without any luck (examples bellow).

For better context, here's example of endpoint that will stream values (as ndjson) upon opening connection:

GET /v2/path/{id}
Accept: application/x-ndjson

Attempt #1:

Issue: The completion handler is never called

let keyID = try keyAdapter.getKeyID(for: .signHash)
let url = baseURL.appendingPathComponent("/v2/path/\(keyID)")

var urlRequest = URLRequest(url: url)
urlRequest.httpMethod = "GET"
urlRequest.setValue("application/x-ndjson", forHTTPHeaderField: "Accept")

session.dataTask(with: urlRequest) { data, response, error in
   // This never gets called.
   // I would expect that the completion is called every time backend emits new value.
}.resume()

Attempt #2:

Issue: Debugger displays this message: Connection 0: encountered error(12:1)

private var stream: URLSessionStreamTask? = nil

func startStream() {
    let keyID = try keyAdapter.getKeyID(for: .signHash)
    let url = baseURL.appendingPathComponent("/v2/path/\(keyID)")

    let stream = session.streamTask(withHostName: url, port: 443)
    // Not sure how to set headers. 
    // Header needs to be set so backend knows client wants to connect a stream.
    self.stream = stream

    stream.startSecureConnection()
    startRead(stream: stream)
}

private func startRead(stream: URLSessionStreamTask) {
    stream.readData(ofMinLength: 1, maxLength: 4096, timeout: 120.0) { data, endOfFile, error in
        if let error = error {
            Logger.shared.log(level: .error, "Reading data from stream failed with error: \(error.localizedDescription)")
        } else if let data = data {
            Logger.shared.log(level: .error, "Received data from stream (\(data.count)B)")
            if !endOfFile {
                self.startRead(stream: stream)
            } else {
                Logger.shared.log(level: .info, "End of file")
            }
        } else {
            Logger.shared.log(level: .error, "Reading stream endup in unspecified state (both data and error are nil).")
        }
    }
}

Does anyone have experience with this? How can I keep HTTP connection open and listen to a new values that backend is streaming?

Solution

iOS can connect to HTTP stream using now deprecated API URLConnection. The API was deprecated in iOS 9, however it's still available for use (and will be in iOS 16 - tested).

First you need to create URLRequest and setup the NSURLConnection:

let url = URL(string: "\(baseURL)/v2/path/\(keyID)")!

var urlRequest = URLRequest(url: url)
urlRequest.setValue("application/x-ndjson", forHTTPHeaderField: "Accept")

let connnection = NSURLConnection(request: urlRequest, delegate: self, startImmediately: true)
connnection?.start()

Notice that the argument for delegate in the code above is of type Any which doesn't help to figure out what protocol(s) to implement. There are two - NSURLConnectionDelegate and NSURLConnectionDataDelegate.

Let's receive data:

public func connection(_ connection: NSURLConnection, didReceive data: Data) {
    let string = String(data: data, encoding: .utf8)
    Logger.shared.log(level: .debug, "didReceive data:\n\(string ?? "N/A")")
}

Then implement a method for catching errors:

public func connection(_ connection: NSURLConnection, didFailWithError error: Error) {
    Logger.shared.log(level: .debug, "didFailWithError: \(error)")
}

And if you have custom SSL pinning, then:

public func connection(_ connection: NSURLConnection, willSendRequestFor challenge: URLAuthenticationChallenge) {
    guard let certificate = certificate, let identity = identity else {
        Logger.shared.log(level: .info, "No credentials set. Using default handling. (certificate and/or identity are nil)")
        challenge.sender?.performDefaultHandling?(for: challenge)
        return
    }

    let credential = URLCredential(identity: identity, certificates: [certificate], persistence: .forSession)
    challenge.sender?.use(credential, for: challenge)
}

There is not much info on the internet, so hopefully it will save someone days of trial and error.